Resilient sheet grippers are widely used in printing presses to non-positively grip a sheet of paper and hold it up against a cylinder. If the paper is pulled out of the gripper even slightly, problems arise with mackling and registration errors occur. Therefore, the gripper typically is required to have a very considerable retaining force, which usually means that the gripper springs must have a very high spring constant. Also, since any play present at the gripper tip would result in registration errors and mackling, it is desirable to minimize the bearing clearances of the gripper elements. The reduced bearing clearance, however, leads to increased friction in the gripper bearings so that some of the spring force operative for gripping is consumed in the bearing itself. The need further arises for the gripper shaft bearings to be very stable in order to reduce deformation associated with the abrupt closure of the grippers. A disadvantage of this is that very high mass forces are produced.
In short, the known gripper systems require very considerable forces for their actuation and only some of such forces can be used for sheet retention. Furthermore, these substantial and abrupt forces may cause unwanted oscillations of the press.
A gripper system of this general kind is shown in DE-PS No. 1 174 804 wherein a one-piece gripper lever is mounted on a gripper shaft with adjustable biasing of the lever provided by a compression spring. One end of the gripper lever is connected to a spring end on a clamping element and the other end bears on the gripper support with the gripper closed and on a bearing stop with the gripper open. The compression spring is so arranged that the horizontal components of the spring force, support force and bearing force act in the same direction. The disadvantage of this type of gripper is that at very high press speeds and at elevated biasings, the gripper reacts sluggishly because of friction between the lever and the gripper shaft and because of the relatively larger masses and inertia radii. Unwanted oscillations introduced into the press also cause the gripper to lose its position when the gripper tip is being centered.
Another known gripper system is disclosed in DE-OS No. 1 908 181 wherein the spindle of the gripper finger pivot is pivotally disposed parallel to the gripper shaft, the spindle of the gripper finger pivot being disposed substantially on the prolongation of a straight line connecting the support surface for the gripper tip to the gripper shaft axis. As is apparent from the geometry shown therein in FIG. 1, the force which the gripper tip applies to the gripper support also has a component in the direction of sheet movement. The sheet may therefore move for this reason and because of possible twisting of the gripper shaft at high biasings, even though there may be some improvement as compared with conventional grippers in which there is an arcuate motion around the gripper shaft axis.
The gripper disclosed in DD-PS No. 67 992 is mounted by means of a clamping member on a pivotable gripper shaft having a stationary axis. A gripper tongue makes a circular movement around such axis in a first movement phase and makes a movement substantially perpendicular to the gripper support in a second movement phase. This gripper, however, uses a non-positive parallel spring strip arrangement and a gripper tongue which cannot withstand substantial closing forces without buckling. The gripper is therefore completely unsuitable for use with very high closing forces.
Gripper systems of the type disclosed in DE-PS No. 2 030 040 utilize a perpendicularly closing gripper with a controlled gripper shaft. A disadvantage of this known system is that the non-positive actuation of the gripper shaft relative to the fulcrum of an actuating lever is by means of a guide on a control cam. The additional components associated with the control cam lead to increased mass forces of the system. The components also oscillate with substantial radii of inertia, leading to a reduction in press performance. Also, if dirt accumulation of the cam is fairly heavy, accurate guidance of the gripper movement phases is impossible.
The gripper system shown in DE-OS No. 3 130 689 uses a soft gripper support along with a gripper finger which has a flat gripper flight path and which closes perpendicularly in the final movement phase. There is a resilient abutment screw disposed in the gripper finger and operative against the sheet gripper stop. A further adjusting screw is needed to adjust the resiliently interconnected holders by which the gripper finger is associated with the gripper shaft. A disadvantage of this type of gripper is that the gripper finger must be associated with a soft gripper support and complicated adjustment must be made by means of two adjusting screws to ensure accurate operation. Furthermore, the gripping action becomes uncertain at high press speeds.